Ignore proximity event blocks with grayed-out sensors

Created by: cwalther

This implements the first part of what I suggested in #446, as a base for discussion: Proximity and button event blocks with all sensors grayed out should never trigger, rather than periodically, and cause a compiler warning.

It turns out that the current version of the VPL compiler is not capable of outputting warnings (while still generating code), only errors (which cause no code to be generated), so that’s what I implemented, as a first step. If there is agreement that that would be desirable, I can extend it to produce warnings instead. (MISSING_EVENT and MISSING_ACTION could be warnings too in my opinion, that would make it easier to test an incomplete program or “comment out” a line by removing its event or action.)

Is there anything I need to do to get the new UI strings internationalized? (I’m unfamiliar with Qt.)

If this is accepted, I will try to implement the second part: add a “periodic timer” block to replace the removed functionality.